Family-based drug use prevention: The «Familias que Funcionan [Families that work]» program.. «Familias que Funcionan»[Families that work] is a family-based drug-use prevention program resulting from the adaptation to the Spanish context of the prestigious North-American «Strengthening Families Program 10-14». The program was applied at four secondary schools (N = 380 pupils) in Asturias (northern Spain). This article presents the results of the assessment of this application after a two-year follow-up, regarding its effectiveness in the reduction of drug use among adolescents and its effects on certain family risk factors. Consistent attendance in the «Familias que Funcionan» program proved effective for reducing both rates (t= -2.73, p<.05 and t= -4.58, p<.005, for the 1- and 2-year follow-ups, respectively) of increase in adolescent drug use and some family risk factors.